Twenty-Four-Hour Energy Expenditure: The Role of Body Composition, Thyroid Status, Sympathetic Activity, and Family Membership*

The present study assessed the possible familial effect in 71 healthy Caucasian siblings on each of the variables determining the inter-individual variations in energy expenditure (EE) measured under standardized conditions. We found that the 24-h EE measured in respiration chambers of 71 siblings from 32 different families was positively correlated with fat-free mass, which explained 82% of the variation between subjects (P < 0.00001). An additional 10% of the total variation was explained by differences in spontaneous physical activity (P < 0.00001), fat mass (P < 0.00001), plasma concentration of free T3 (P < 0.003), and norepinephrine (P < 0.002), whereas plasma values of epinephrine and androgen hormones did not correlate with 24-h EE. After adjustment for gender, there was a familial aggregation of both 24-h and sleeping EE, as indicated by intraclass correlation coefficients (r) of 0.44 (P < 0.02) and 0.58 (P < 0.01), respectively. The familial effect on gender-adjusted 24-h EE was explained mainly...